From Hans message on the mailing list:
Another addition is that
define an undefined mode; the keep makes sure that the already set value is kept (another option is 'yes').
Using defined modes (that is, set with: \enabledmode, \disablemode or \definemode) can be tested about twice as fast as undefined modes which can make a small difference when modes are tested lots of times.
Add to the main article with appropriate example --Aditya 20:20, 27 June 2014 (CEST)